=opacity( !pct )
  :opacity= !pct/100
  :-ms-filter "progid:DXImageTransform.Microsoft.Alpha(Opacity=#{!pct})"
  :filter alpha(opacity='#{!pct}')

=user-select( !user_select )
  :-moz-user-select= !user_select
  :-khtml-user-select= !user_select
  :-webkit-user-select= !user_select
  :user-select= !user_select
  
// from http://gist.github.com/215561
// Apply a CSS text-shadow cross-browser.
// There is one todo/gotcha: the direction is hardcoded to towards 
// the bottom-right for IE, but that's good enough for my use case.
// Usage example: +text-shadow(#333, 1px, 1px, 1)

=text-shadow(!color, !x, !y, !blur)
  :text-shadow= !color !x !y "#{!blur}px"
  :-ms-filter "progid:DXImageTransform.Microsoft.Shadow(Color=#{!color},Direction=135,Strength=#{!blur})"
  
=background-size( !x, !y )
  :-moz-background-size= !x !y
  :-khtml-background-size= !x, !y
  :-webkit-background-size= !x, !y
  :-o-background-size= !x, !y
  :background-size= !x, !y